I have a 24v trolling motor set up. There are 2 batteries that I would call normal size, they are 31's. The 3rd battery is much bigger. What is the bigger battery? It is not charging any more and I need to replace it. Can it be replaced with a 31 like the 2 other ones that I have or do I need to get that big one again?

Why don't you post a picture of the setup. Generally, you'll have two batteries on one side wired together straight to your trolling motor. Then you'll have a third which should be wired to your motor and accessories. Could it be wired to a switch between your trolling motor batteries and it so you can flip it to one of the trolling motor batteries if it went dead and use one of them to crank the motor?

If it is your cranking battery, you need to look at the cranking power and capacity to decide what you want. You need enough capacity to run any fishfinders, livewells, bilge pumps, lights, etc. and still have enough power to crank your motor.

I split mine up so that the engine starting battery only cranks and provides power to the tach, fuel gauge and hour meter. Trolling motor runs off a 24V bank along with the electronics and lights. I come off the first battery to get the 12V to run the console. That was by my design. I can always get home without lights or stereo, but I don't want anything drawing off of the start battery. My instrument lights operate off the nav light switch. I don't need lights on the instruments until I need running lights to run at night.